In recent years, a laser-type optical chemical-species sensor has been developed to deal with problems related to extraction measurement techniques. The laser-type measurement techniques can perform measurement “on site”, and also has another advantage, which is a high-speed feedback suitable for a dynamic process control. The tunable diode laser absorption spectroscopy (TDLAS) is a particularly promising technique for measuring the composition and temperature of combustion gas and other combustion parameters. TDLAS is typically performed by a diode laser which operates in the near-infrared spectral region and the mid-infrared spectral region. Suitable lasers have been developed extensively to be used in the long-distance telecommunication industry, and are easily accessible for the TDLAS usage. Various TDLAS techniques have been developed, which are more or less suitable for detection and control of a combustion process. Commonly known techniques include the wavelength modulation spectroscopy, the frequency modulation spectroscopy, and the direct absorption spectroscopy. In each of the above techniques, light is conducted through a combustion process chamber, and is absorbed in a specific spectral band that uniquely corresponds to a gas that exists in the process chamber, or a combustion chamber, before being received by a detector. It is based on a pre-obtained relationship between the amount and the characteristics of laser light. The absorption spectrum received by the detector is used to obtain the amount of a gas chemical species to be analyzed, and a combustion parameter (e.g. temperature) related to the amount.
Furthermore, generally, TDLAS is performed by transmitting laser light that has passed through a target environment, and then detecting absorption of the laser light at a specific wavelength due to a target gas such as carbon monoxide and oxygen. The spectrum analysis on the detected light makes it possible to identify the type and amount of gas along a laser path. The laser absorption spectroscopy is contact-free, and thus is suitable for use in a severe environment, such as a combustion section in a coal combustion power plant, and a flammable or toxic environment where other types of probe cannot be used. While such an environment may cause extreme attenuation (typically, light loss over 90%), using laser light makes it possible to achieve a high luminance required to realize transmission which can be detected even under such attenuation. To bear the harsh condition of a target usage more suitably, laser light may be sent into a target environment through protected optical fiber (e.g. Patent Document 1).
Further, Patent Document 2 discloses a gas concentration measuring apparatus that uses the laser absorption spectroscopy, which includes a light source part capable of adjusting laser oscillation wavelengths to a plurality of absorption wavelengths inherent to a gas to be measured, a modulation part for applying modulation to the laser oscillation wavelengths emitted from the light source part and outputting a reference signal synchronized to the modulation, a light receiving part for receiving the laser light having passed through the target gas in a measuring region and outputting signals corresponding to the intensities of the received beams, a phase sensitive detection part for detecting the component synchronized to the modulation signal added to the laser light or the higher harmonic component from the signal of the light receiving part on the basis of the reference signal outputted from the modulation part and outputting the same, and an analyzing part for calculating the concentration of the target gas in the measuring region and the concentration of solid particles on the basis of the signal outputted from the phase sensitive detection part and the signal from the light receiving part.
Patent Document 3 discloses a laser type gas analyzer for receiving a laser light in a mid-infrared region including an optical absorption spectrum of a measurement target gas, and calculating gas concentration from an amount of change of a signal component affected by optical absorption by the measurement target gas. The laser type gas analyzer includes: a near-infrared laser emission part for emitting a laser light of a near-infrared region including an optical absorption spectrum of water being present in an inside of a flue without including the optical absorption spectrum of the measurement target gas; an optical system such as a lens for irradiating the inside of the flue with a near-infrared laser light; a lens and a near-infrared photodetector for receiving the near-infrared laser light and outputting it as an electric signal; a water concentration calculation part for extracting a signal component affected by optical absorption by water being present in the inside of the flue from the electric signal and calculating water concentration from the amount of change therein; and a gas concentration correction part for correcting a concentration measurement value of the measurement target gas calculated by using mid-infrared laser light on the basis of the water concentration.